Emission spectrometers are useful for both qualitative and quantitative analyses. Typically, a spectrometer comprises: an energy source, such as a plasma jet, into which the sample to be analyzed is introduced to provide a spectral energy distribution of the elements characteristic of the sample; an optical resolution system to distribute and display the emission spectra onto a focal plane; and a detector system to detect and measure the radiant intensity and wave length of the observed spectra, whereby the identity and, if desired, the amount of each element in the sample may be determined. Optionally and conventionally, the spectrometer system may also contain various processor systems for processing the information received by the detector, and a display system to display or print out the processed results.
One spectrometer device known as an echelle-prism spectrometer, which provides for the focusing of the horizontal and vertical components of the spectra at the exit focal plane of the spectrometer, is described in U.S. Pat. No. 3,658,423, issued Apr. 25, 1971, hereby incorporated by reference.
A variety of different means may be used to detect the spectra; however, in most spectrometer systems, the detectors are photomultiplier tubes (PMT), since at present the PMT provides the best signal/noise ratio (S/N) for the detecting of elements; for example, in the 2000 to 8000 A spectral region. Often, two PMTs of different range of sensitivity are used to cover the desired spectral range.
Detection may comprise the use of a single PMT in a sequential mode, wherein each element in the spectra to be analyzed is detected individually and sequentially in time, with adjustments made in the exit slit each time to optimize the S/N ratio and the sensitivity of the spectrometer. Detection may also comprise the employment of a plurality of PMT for simultaneous multielement detection. In the multielement detection system, an array of PMT is employed behind the focal plane of the spectra, with the single slit in the focal plane replaced by a fixed pattern of exit slits. The fixed pattern of exit slits provides a fixed average optimum position for the number (N), for example, up to 20 spectra lines of interest. The exit slits positioned in the focal plane correspond to wave length characteristics of the elements to be detected.
One multielement system employs a cassette which has a fixed pattern of exit slits in the face panel of the cassette relative to the elements to be determined, which face panel is placed in the exit focal plane of the spectrometer. The spectral energy proceeds through a direct channel in the cassette and impinges directly on an aligned PMT on the other side of the cassette, with typically one PMT used per cassette channel. The signals of the PMTs are then compiled, processed and displayed.
The multielement system is satisfactory for the detection of elements which are characterized by widely divergent emission spectra and exit slits; for example, elements like gold and silver. However, the multielement system provides a significant problem wherein the elements to be detected give spectral emission lines which have exit slits and wave lengths closely spaced together, such as, for example, in the detection of mixtures containing boron, tungsten and iron and many other combinations of elements.
PMTs have a finite size and, where spectral lines are close together, PMTs cannot be stacked or arranged together beyond such size limitations. Accordingly, where the spectral lines are close together as, for example, less than 100 or 10 Angstrom (A) units apart, separate PMTs cannot be positioned and aligned to receive the spectral energy of such closely positioned elements, particularly with the echelle spectrometer of U.S. Pat. No. 3,658,423. Thus, the multielement system cannot be used to detect such elements with PMTs with a multielement cassette. Therefore, there exists a need for an effective spectometric system and cassette and method of operation to permit the simultaneous spectrometric detection and determination of elements in a sample where the element spectral lines are close together.